In general, exhaust gas of a compression ignition type internal combustion engine (diesel type internal combustion engine) often contains so-called PM (particulate matter) formed mainly of carbon (C). This PM becomes a cause of deterioration of visibility and deposition of stain on buildings. For this reason, many internal combustion engines provide their exhaust passages with particulate filters (hereinafter referred to as “filters”) etc. so as to trap the PM in the exhaust gas, and burn off the PM trapped on the filters by adding fuel to the exhaust gas, and burning the added fuel on the filters, or by using heaters to heat the filters, etc. to make the filters high in temperature.
However, if raising the temperature of a filter to burn off the PM in the exhaust gas in this way, the amount of fuel consumption increases with both the methods of adding fuel and heating by a heater and therefore deterioration of the fuel economy is invited. Therefore, in recent years, note has been taken of the point that ozone enables PM to be burned off even at a low temperature, and methods of using plasma to generate ozone and running the generated ozone into the filters etc. to remove the PM trapped on the filters etc. have been proposed.
Japanese National Patent Publication (A) No. 2005-502823 discloses an exhaust gas purification system providing a plasma generator at an upstream side of a filter in the exhaust, using this plasma generator to generate ozone or nitrogen dioxide, running the generated ozone or nitrogen dioxide into the filter, and burning the PM trapped on the filter. In this exhaust gas purification system, when the temperature of the exhaust gas is lower than a certain temperature threshold value (for example, 200° C.), the plasma generator mainly generates ozone, while when it is that temperature threshold value or more, the plasma generator mainly generates nitrogen dioxide. This is done because the power consumed when generating ozone is larger than the power consumed when generating nitrogen dioxide, so ozone is only generated in a temperature range where the ability of nitrogen dioxide to oxidize the PM is low, that is, a temperature range lower than the above temperature threshold value.
In this regard, the temperature of the exhaust gas in a compression ignition type internal combustion engine is a comparatively low. States of a temperature lower than the temperature threshold value continuing are not uncommon. For this reason, in the exhaust gas purification system, in many cases, the plasma generator generates ozone to burn off the PM trapped on the filter and therefore the power consumption becomes great.
Further, if trying burn off PM by ozone after a certain degree of PM has been trapped by the filter, a large amount of ozone would have to be generated in a relatively short time. If trying to generate a large amount of ozone in a short time, not only will the power consumption become large, but also the plasma generator will have to have a high ozone generating ability which will therefore end up inviting an increase in production costs.